1. Field of the Invention
The present invention relates to an image analysis device and method and a storage medium having stored therein a program which analyze a radiographic image of a subject obtained by radiography, and more particularly, to an image analysis device and method and a program which analyze a radiographic image of a subject obtained by radiography to estimate the thickness of the subject at each position of the radiographic image.
2. Description of the Related Art
It is known that, when a radiographic image of the subject is captured with radiation passing through the subject, the influence of the scattering of radiation or a reduction in the transmissivity of radiation in the subject increases as the thickness of the subject increases, which results in a change in the quality of the acquired radiographic image. Therefore, a technique has been proposed which roughly estimates the thickness of the subject, on the basis of various kinds of information, such as imaging conditions, a signal value of a radiographic image, the histogram width of the signal value of the radiographic image, and the length of the subject in the subject image in a predetermined direction and changes the conditions of image processing, such as a process of removing scattered X-rays of the captured radiographic image, or imaging conditions applied to the capture of the radiographic image, on the basis of the estimated thickness of the subject.
For example, JP1990-244881A (JP-H02-244881A) discloses a method which measures the pixel value of an image of a simulated subject with a known thickness, which is captured by radiography under known imaging conditions, prepares an association table in which the body thickness is associated with the pixel value in advance, estimates a rough body thickness distribution according to the pixel value of the subject image, on the basis of the association table, estimates a scattered X-ray component of the subject image corresponding to the body thickness distribution of the subject image, and subtracts the scattered X-ray component from the subject image to acquire a processed image.
Trotter et al., “Thickness-dependent Scatter Correction Algorithm for Digital Mammography”, Proc. SPIE Vol. 4682, May 2002, pp. 469-478 discloses a method which estimates a scattered X-ray component of a radiographic image on the basis of a thickness distribution of the human body and removes the scattered X-ray component. According to the image processing method disclosed in Trotter and four others, “Thickness-dependent Scatter Correction Algorithm for Digital Mammography”, Proc. SPIE Vol. 4682, May 2002, pp. 469-478, an estimated scattered X-ray image obtained by applying a predetermined function to an input subject image and estimating an image of a scattered X-ray included in the subject image is generated on the basis of the body thickness distribution estimated from the pixel value of the subject image and the estimated scattered X-ray image is subtracted from the subject image to generate an estimated primary X-ray image obtained by estimating a primary X-ray image from the input subject image. In addition, a process of applying a predetermined function to the generated estimated primary X-ray image to generate an estimated scattered X-ray image and subtracting the estimated scattered X-ray image from the subject image to generate an estimated primary X-ray image is repeated until the estimated scattered X-ray image is converged under predetermined convergence conditions, the converged estimated scattered X-ray image is calculated, and the estimated scattered X-ray image is subtracted from the subject image to finally acquire a processed image from which the scattered X-ray component has been removed. In addition, Trotter and four others, “Thickness-dependent Scatter Correction Algorithm for Digital Mammography”, Proc. SPIE Vol. 4682, May 2002, pp. 469-478 discloses a method which adjusts a predetermined function for estimating the image of the scattered X-ray included in the subject image on the basis of the body thickness.